A variety of electrical connectors are used throughout the automotive glazing industry. For example, T-piece connectors may be used to connect an electrically conductive circuit, such as a circuit printed on the surface of a ply of glass, or an array of wires fixed within a laminated glazing, to the wiring harness of a vehicle. Such circuits generally find use as heating circuits, to promote de-misting or de-icing, or antenna circuits. The T-piece connector is soldered to an electrically conductive substrate known as a bus bar, which may be provided directly on the surface of a piece of glass, or fully or partly on a fired, printed band on the glass, known as an obscuration band. The bus bar is typically printed using a silver-containing ink. Traditionally, the solder used to join the bus bar and the connector contains lead. However, lead is known to be harmful, and there is increasing pressure to use lead-free solders in the automotive industry, for example, such as that described in WO2004/068643. WO2004/068643 discloses tin-based solders (up to 90% by weight tin), comprising a mechanical stress modifier selected from bismuth or indium. The solder may also contain silver and/or copper.
However, one disadvantage to using a lead-free solder is that the adhesion between the connector and the bus bar may not be as high as that given by a lead containing solder. One solution to this is to additionally use an adhesive to bear the mechanical load on the connector, as in EP 1 256 261.
Even if additional adhesive means are used, the adhesion between the connector and the bus bar due to the solder will degrade over time due to environmental conditions, such as extremes of temperature and humidity. Once the joint between the connector and the bus bar starts to degrade, the adhesion decreases and the connector may become loose or detach from the bus bar altogether. In addition, when the joint deteriorates, the quality of the electrical connection achieved may become poor. This degradation may be quicker if there are regions of poor adhesion due to air bubbles or debris at the surface where the connector and bus bar are joined. To improve the reliability of the joint therefore, great care must be taken not only in adhesive and solder selection, but with processing techniques and conditions.